There are already covers for sheathing stalks of bananas, consisting of a plastic film in polythene (polyethylene), that enable protection to be provided for the stalk of bananas against insects and against the rubbing of leaves and fruit due to conditions such as bad weather, dust, etc..
But bananas are also sensitive to ultra-violet "B" (UV "B") radiation (280-320 nm) which results in the appearance of blemishes on the banana's skin (black marks) and which causes a disappearance of the chlorophyll (chlorosis). Bananas are also sensitive to infrared (heat) which can cause the same symptoms when the temperature of the fruit exceeds 35.degree. C.
A blue colorant (optalocyanyl blue, ultramarine blue) was then proposed which, introduced into the polythene sleeves, protects the bananas against burns.
However, this blue cover does not allow all the white light to pass and stops a good part of the UV "A", because of its blue colouring. Nevertheless white light and UV "A" are involved in healing injuries due to UV "B". The wavelengths absorbed by the blue colorant are those absorbed by the chlorophyll and are those that the plant needs. In addition, polythene is a material with a negative greenhouse effect that lets all infrared pass: the bananas are thus heated by day and rapidly cooled by night. An increase in the heat transmitted to the stalk at night enables growth to be accelerated by favouring reactions in the dark.